1. Field
This specification relates to layered semiconductor structures and more particularly, to light emitting devices incorporating layered III-V alloy semiconductor structures for use in durable laser diodes having light emissions of longer wavelengths, and apparatus and system using such light emitting devices.
2. Description of the Related Art
As communication systems have developed and both desired and current information transmission rates have increased, more attention has been focused on the development of optical communication systems.
As presently contemplated, the optical communication system presently used in main communication lines will be extended to each subscriber's domestic line. To implement such systems, it is indispensable to develop smaller and less expensive optical devices such as, among others, light emitting devices like laser diodes and light emitting diodes and photoreceptors.
Light emitting devices such as laser diodes are conventionally accompanied by a cooling device such as a peltier element or heat sink so as to control the change in device temperature caused by input current. However, it would be highly desirable to provide stable laser diodes which do not need cooling devices, in order to widely implement light emitting devices in the communication system.
To achieve stable operation of the laser diodes at practical operation temperatures, it is desirable that these devices have improved device capabilities such as relatively low threshold current density and reduced temperature variation in device characteristics.
When a layer of semiconductor is formed on a semiconductor substrate having lattice constants different from each other, it is known that the former can generally be grown with satisfactory crystalline quality up to the critical thickness, which is the thickness above which undesirable strain-induced dislocation appears. This critical thickness may be calculated from the magnitude of strains induced by the difference in lattice constants of these semiconductor materials.
However, in the previous semiconductor alloy systems such as, for example, the combination of GaInPAs as the layer material and InP as the substrate (or GaInPAs/InP), there has been found no appropriate combination to attain a desirable magnitude of the conduction band discontinuity, thereby leading to difficulty in attaining satisfactory high temperature device characteristics.
Several semiconductor laser diodes have been proposed to improve temperature characteristics.
As an example, a ternary GaInAs substrate is disclosed to grow active layers thereon, of a semiconductor light emitting device in Japanese Laid-Open Patent Application No. 6-275914. Since the substrate is composed of GaInAs, on which a semiconductor material may be grown having a wider band gap energy, this may lead to the formation of a desirable magnitude of the conduction band discontinuity, which has not been achieved previously by the InP substrate.
As another example, a semiconductor laser device on a GaAs substrate is disclosed in Japanese Laid-Open Patent Application No. 7-193327 to attain long wavelength emissions. The laser device comprises a GaAs substrate, on which are grown a GaInAs buffer layer for relaxing the lattice with a lattice constant larger than the substrate and active layers further thereon.
Since a semiconductor layer is grown on the GaInAs substrate, having a lattice constant larger than that on the GaAs substrate, a larger value of the conduction band discontinuity may be attained similarly to the above-mentioned light emitting device disclosed in the '914 patent Application.
In addition, a semiconductor layer can be grown on a GaAs substrate, having a band gap energy larger than that on other substrates such as those composed of InP and ternary GaInAs.
However, in the previous semiconductor alloy systems, there has been found no such semiconductor material serving as an active region which has a band gap energy corresponding to longer wavelength emissions such as, for example, the 1.3 micron region. Namely, when the GaInAs layer is grown on the GaAs substrate, the emission wavelength from the GaInAs layer increases with the increase in the In content. However, this increase in In content leads to an increase in the amount of lattice strains. In addition, since the critical value of lattice strains is approximately 2%, it has been suggested that this value yields a limit to the emission wavelength to approximately 1.1 micron according to IEEE Photonics Technol. Lett. Vol. 9, pages 1319-1321 (1997). Photo-emission for the material, which is grown at 400° C. by molecular beam epitaxy (MBE) method, is observed at the wavelength of 1.223 micron (Journal of Applied Physics, Vol. 78, pages 1685-8 (1995)). This material, however, has not been used in a practical semiconductor laser device.
A further laser diode comprising a GaInNAs active layer formed on a GaAs substrate has been disclosed in Japanese Laid-Open Patent Application No. 6-37355. In that disclosure, GaInAs layers, which have a lattice constant larger than that of GaAs, are added with nitrogen (N) to thereby form GaInNAs layers having a decreased lattice constant, to thereby be lattice-matched to GaAs.
In addition, since N has an electronegativity larger than other elements, band gap energy of the GaInNAs layers further decreases. As a result, light emissions at the wavelength region of 1.3 μm or 1.5 μm have become feasible in these devices.
As another example, calculated results on the energy level line-up are described by Kondow et al. in Japanese Journal of Applied Physics, Vol. 35, pages 1273-5 (1996), for a laser diode comprising a GaInNAs active layer formed on a GaAs substrate.
It is described in this publication that, since the GaInNAs system is lattice-matched to GaAs, cladding layers may be provided which are composed of AlGaAs, which has a larger band gap energy, in place of the materials which are lattice-matched to GaAs. In addition, by the addition of N, the energy levels of both valence and conduction bands decrease as well as the band gap energy for the hetero-junction which is formed between the GaInNAs active layer and cladding layer. Therefore, a large value of the conduction band discontinuity may be attained. This allows fabrication of laser diodes having improved temperature characteristics.
Also, in order to attain conduction band discontinuity with an appropriate magnitude to prevent the overflow of carriers and satisfactory high temperature characteristics of long wavelength lasers in a similar manner to GaInNAs formed on GaAs, InNPAs layers formed on InP substrates have been disclosed, including one by the inventor in Japanese Laid-Open Patent Application No. 10-126005, and another in Japanese Laid-Open Patent Application No. 9-219563.
Other examples have also been disclosed on GaInNAs laser structures, including edge emitting laser structures in Japanese Laid-Open Patent Applications No. 8-195522 and 10-126004, and surface emission type devices in No. 9-237942 and 10-74979.
Several 1.3 micron range GaInNAs lasers have been fabricated recently, such as one incorporating a double-hetero structure comprising a thick GaInNAs active layer formed on, and lattice-matched to, a GaAs substrate with 3% of N and 10% of In, according to Electron Lett. Vol. 33 pages 1386-71 (1997); and the other incorporating a compressedly strained GaInNAs single quantum well structure with 1% of N and 30% of In according to IEEE Photonics Technol. Lett. Vol. 10 pages 487-88 (1998). A laser emission is observed at the wavelength of 1.1168 micron for the latter material, which has 30% of In with no N and which is incorporated into a quantum well structure having a well thickness of 7 nm. This emission has the longest wavelength, to our knowledge, which is observed at room temperature for the compressive strained GaInAs quantum well active layer grown on the GaAs substrate.
However, some of the materials and systems described above have several shortcomings which follow. Namely, the ternary GaInAs substrate disclosed in the Application '914 is difficult to form, the laser structure incorporating the GaINnAs buffer layer disclosed in the Application '327 has a shortcoming in its durability, and the III-V alloy system which includes N or other group-V element, such as GaInNAs, tends to grow having a resulting crystallinity considerably degraded with the increase in N content.
As for the methods of growing a III-V semiconductor alloy layer, which includes N and other group-V elements, there are found several disclosures including the metal organic chemical vapor deposition (MOCVD) method disclosed in Japanese Laid-Open Patent Application No. 6-37355, the MOCVD method using dimethylhydrazine as the N source disclosed in Japanese Laid-Open Patent Applications No. 7-154023 and 9-283857, and the MBE method using active N species disclosed in Japanese Laid-Open Patent Applications No. 6-334168.
In these growth methods, however, there have been realized several shortcomings such as, among others, difficulties in incorporating an appropriate amount of N into the III-V semiconductor alloy layers because of a small rate of the N inclusion into the layers during the layer growth.